1. Field of the Invention
The present invention relates to a positively chargeable toner that is used for an electrophotographic method.
2. Description of the Related Art
With respect to the positively chargeable toner, a pulverized toner, which is obtained by subjecting, for example, a binder resin such as a styrene-acrylic-based resin, a nigrosine dye, a positive-charge controlling agent such as a quaternary ammonium salt or triphenyl methane and a wax for preventing offset to fusing/kneading/pulverizing processes, has been conventionally known. In order to impart sufficient positive chargeability to the toner of this type, a positive charge controlling agent is normally added thereto at a rate of about 3 to 7 parts by weight with respect to 100 parts by weight of the binder resin. However, in the case when the positively chargeable toner of this type is applied to a two-component developer that is used together with magnetic carrier, spent positive charge controlling agent is generated on the surface of the magnetic carrier within repetitive use for a long time, resulting in degradation in the frictional charging function (toner chargeability) of the carrier to the toner and the subsequent toner fogging on a non-image portion of an image. The nigrosine dye is often used as the charge controlling agent; however, such a dye-based charge controlling agent has a blackish color, and is not suitable for color developing processes.
In order to solve the above-mentioned problem, in the pulverizing method, a technique in which, in place of the positive charge controlling agent, for example, a positive charge controlling resin is added has been proposed (for example, see Unexamined Japanese Patent Publication No. 10-333358). However, since the toner is manufactured through a pulverizing method and since the added amount of the wax is subsequently limited, it is not possible to sufficiently prevent offset, resulting in problems in the toner fixing property. When the added amount of wax is increased so as to improve the fixing property, the wax tends to be exposed to the toner surface, resulting in problems with the toner chargeability.
In this manner, the disadvantage of the positive chargeable pulverized toner is that it is difficult to make the fixing property and the charging property well balanced with each other, and this forms a major problem with the pulverized-type toner.
In recent years, there have been strong demands for an oil-less system and an energy-conserving apparatus in the fixing system, and there have been many proposals in which these functions are imparted to wet-type polymerization toners. The wet-type polymerization toner refers to a toner manufactured through a wet-type polymerization method such as a so-called emulsion polymerization method and suspension polymerization method, and with respect to the toner of this type, more wax can be added to the toner to a certain extent in comparison with the pulverized toner, without causing adverse effects on the chargeability.
At present, among these demands, in an attempt to achieve an energy-conservation fixing process, many companies are vigorously trying to design low-temperature fixing systems, and many related novel techniques have been proposed; however, these techniques have the following disadvantages.
In an attempt to achieve toner low-temperature fixing processes, there are mainly two approaches. That is, one is to provide a resin with lower viscosity, and the other is to lower the melting point of the wax to further increase the added amount. However, the common problem with these methods is to cause a great reduction in the viscosity of the toner. Subsequently, problems such as a reduction in the resin viscosity, degradation in the heat resistant storing property and the anti-blocking property due to a viscosity reduction in the resin and a lowered melting point of the wax, and deterioration in the charging property and cleaning property due to an increased amount of wax, have been raised.
These problems can be solved to a certain extent by modifying the toner structure into a capsule structure and a core-shell structure. For example, in a styrene-acrylic copolymer composition that can be manufactured by an emulsion polymerization method, the molecular weight of the inner layer is made smaller, while the molecular weight of the outer layer is made greater, so that the viscosity of the toner outer shell layer is improved, thereby making it possible to improve the heat resistant storing property and the anti-blocking property of the toner. However, even when such a toner structure is used, this solution is still dependent on the toner viscosity and the resin is designed based upon the balance between the viscous component and the elastic component; therefore, in the case when the designing is carried out with a view to improving the low-temperature fixing property, the elastic function is lowered and the fixing separation property and the anti-offsetting property at the time of an oil-less fixing process are seriously lowered.
Even in an attempt to carry out the controlling process based upon the melting-point and added amount of the wax without adjusting the visco-elastic function of the resin, since the visco-elastic function of the toner is changed in the same manner as the resin, problems arise with respect to the visco-elastic function controls of the resin.
Therefore, at present, in order to achieve a low-temperature fixing process, the conventional toners need to sacrifice the other properties (such as fixing separation property and anti-offset property) of the fixing properties.